Method for radiography



Patented Apr. 13, 1954 UNITED PATENT OFFICE nQ-prawm a lication April 28, 1952, Serial No. 284g848= 2 Claims. (01. 25065) The invention concerns method and apparatus for radiography wherein radioactive isotopes of an activity less than 200 kilovolts may be employed regardless of the number and activity of radiations of the isotope other than the low gamma radiation existing at a value less than the 200 kilovolts specified.

I have found a method of operation wherein multiple radiations may be eliminated or avoided so that a remaining low energy gamma radiation can be employed for radiography purposes especially for medical diagnostic work.

I have found the following isotopes to be most useful: cerium-praeseodymium 144, europium 155, thulium 170, terbium 160, indium 114, and osmium 193-485. The first two of these are greatly to be preferred as they are of the class of isotopes known as fission products. These materials are produced in large quantities from the normal operation of nuclear reactors so their commercial availability is assured. Also they can be concentrated into sources of virtually unlimited specific activity (that is, very large amounts of activity in very small physical size). The ultimate limit to specific activity is 100% of the atoms to be radioactive.

Effects of beta radiation may be avoided according to the invention by employing a filter such as a plastic material or light-weight hydrocarbon having an affinity for absorption of beta radiation without accompanying absorption of gamma radiation. In particular, this invention envisages a filter of such material that the effects of bremstrahlen rays are reduced to a minimum. For instance, in the use of ceriumpraeseodymiurn 144 the beta radiations, by their interaction on surrounding matter, are liable to produce bremstrahlen which are in the nature of X-rays with a maximum energy of 3000 kilovolts. To avoid the production of such undesirable radiation, it is necessary that material immediately adjacent to the radioactive source be of as low an atomic number as possible. A filter of a plastic material or hydrocarbon accomplishes this, and other materials such as beryllium compounds would also be practicable as filters. In this connection, it must be noted that the source itself must be as free as possible from any material of high atomic number. This is especially significant with regard to fission products which are normally prepared by the use of a carrier material. For example, in the case of cerium-praeseodymium 144, this material must be precipitated together with such compounds as beryllium hydroxide, lithium fluoride, and other materials of low atomic number.

In company with a beta radiation filter of this kind, I may employ a particular fluorescent material adjacent a film face to be exposed and which has an activity characteristic exclusive of the multiple radiations but acceptable to the particular gamma radiation with which I am concerned, namely, that existing at an energy level under 200 kilovolts. In some cases it may not be necessary to employ a fluorescent screen material of this kind. For example, in employing cerium-praeseodymium 144 as a source, it is possible to obtain a picture on a photographic plate without the use of a special screen in cases where the material of the emulsion or the plate itself presents a different absorption coefficient for the different radiations of the source. Thus, the low energy radiation may be appreciably absorbed by the emulsion or film material whereas the higher energy radiations may pass through the film or emulsion losing a very small amount of energy therein and, accordingly, the film image substantially represents the pattern developed by the incidence of the low energy radiation.

The low energy gamma radiation may be produced either directly or by internal conversion and it is of great significance that the presence of a certain amount of high energy gamma radiation above 200 kilovolts may be accepted in the source for use in accordance with the invention.

Example A picture of the structure of the hand was made on a Kodak no-screen photographic plate. A source of cerium-praeseodymium 144, 12 inches away with a filter of Lucite of thickness next the source and a 15 minute exposure were employed. The source was of 150 millicuries and square inch projected area although a pin point source at curies intensity or equivalent is preferred.

While the results obtained do not demonstrate the full contrast which may be obtained by X-rays, almost all the details of bone structure are clearly revealed and many more would be revealed with a pin point source. The comparative lack of contrast may, in many cases such as the setting of fractures, be an advantage since the position of the soft tissue and of the bone are revealed on the same radiograph.

The invention thus contemplates sources of the character disclosed employed in combination with a radiation sensitive screen either of the fluorescent type or sensitive emulsion type, such as employed in photographic plates. It is particularly contemplated that the source of the in 3 vention shall be employed in combination with a photographic film.

What I claim as my invention is:

1. A radiographic method of forming an image of a radiation source on a screen comprising: selecting an isotope gamma radiation source having low energy gamma radiation of a value below 200 kilovo lts; selecting a screen material having a high absorption for the low energy radiation as compared with any high energy radiation of the source; subjecting the screen to radiation from said source; and converting substantially only the low energy radiation absorbed by the screen into a visible image.

2. A radiographic method of forming an image of a radiation source on a screen, comprising: selecting an isotope gamma radiation source having low energy gamma radiation of a value below 200 kilovolts; selecting a lightweight hydrocarbon screen material having a high absorption for the low energy radiation as compared with any high energy radiation of the source; subjecting the screen to radiation from said source; and converting substantially only the low energy radiation absorbed by the screen into a visible image.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,257,143 Wood Sept. 30, 1941 2,407,381 Pecher Sept. 10, 1946 

1. A RADIOGRAPHIC METHOD OF FORMING AN IMAGE OF A RADIATION SOURCE ON A SCREEN COMPRISING: SELECTING AN ISOTOPE GAMMA RADIATION SOURCE HAVING LOW ENERGY GAMMA RADIATION OF A VALUE BELOW 200 KILOVOLTS; SELECTING A SCREEN MATERIAL HAVING A HIGH ABSORPTION FOR THE LOW ENERGY RADIATION AS COMPARED WITH ANY HIGH ENERGY RADIATION OF THE SOURCE; SUBJECTING THE SCREEN TO RADIATION FROM SAID SOURCE: AND CONVERTING SUBSTANTIALLY ONLY THE LOW ENERGY RADIATION ABSORBED BY THE SCREEN INTO A VISIBLE IMAGE. 